How should I encode the input which are 5 cards from a deck of 52 cards?
Depends on how your game is played. Is there any meaning assigned to the order of cards, or are all 5 played simultaneously? If order matters, use 5 one-hot vectors so you can choose how to order them, otherwise use a single 5-hot input vector. I would also add that if temporal order matters, you could also use a recurrent net with a 52-element input and feed the five one-hot vectors one after another.